This invention relates to a liquid vaporizer-feeder. More specifically, this invention relates to an improved vaporizer-feeder capable not only of supplying a source liquid accurately but also of accurately controlling the transportation of liquids such as alcohols and organic acids that are widely used in chemical industries. Such a liquid vaporizer-feeder is useful in a semiconductor fabrication process, for example, for accurately controlling the flow rate of tetraethyl orthosilicate (TEOS) for the formation of a thin film and, in particular, in the case where the vaporized liquid is transported to a reaction chamber in a reduced-pressure condition.
To explain the background of the present invention by way of a chemical vapor deposition (CVD) process in the production of a semiconductor device, it is to be noted that TEOS is recently coming to be seriously considered as a layer-to-layer insulating film material for semiconductor wafers. One of the reasons for this choice is its superior step coverage because the rate determination is by surface reaction. This is to be contrasted to the deposition mechanism by SiH.sub.4 with the conventional reduced-pressure CVD. Another reason is that SiH.sub.4 is extremely reactive and there is a relatively high probability of an explosion. By contrast, TEOS is safer and easier to store, and its cost, as a source material, is expected to become lower in the future.
Examples of CVD method using TEOS include the reduced-pressure CVD, the normal-pressure CVD and the plasma CVD. If a reaction chamber is used at a normal pressure as in the case of a normal-pressure CVD method, the pressure inside the pipes should be raised higher, as one moves farther upstream from the reaction chamber because, otherwise, the source liquid would not flow through the piping. In the case of the reduced-pressure or plasma CVD method, the source liquid naturally flows into the reaction chamber because the reaction chamber is in a reduced-pressure condition.
FIG. 6A shows a schematic diagram of a system for using TEOS, including a reaction chamber 50' and a prior art liquid vaporizer-feeder (liquid mass flow controller) 40' of a type which has been in common use, comprising a sensor tube 1', a bypass tube 2', a flow rate control valve 7' and a vaporization valve 12' which introduces a carrier gas H' (with or without a reaction gas R') and sends our a mixed gas Kn'. The flow rate control valve 7' is unitarily connected to a housing 16' which contains the sensor tube 1' and the bypass tube 2'. The flow rate control valve 7' is also connected through an elongated connector tube 23' to the vaporization valve 12' which is set inside a thermostat 45'.
Assume now that the reaction chamber 50' is operating under a normal-pressure condition. FIG. 6B shows the pressure on the source liquid L' in this system. If we further assume that P.sub.2 =1 kg/cm.sup.2 and P.sub.1 =2 kg/cm.sup.2, the pressure difference .DELTA.P is 1 kg cm.sup.2. In other words, a pressure difference of about 1 kg/cm.sup.2 is generated across the connector tube 23', and, as the liquid L' approaches the vaporization valve 12' by flowing through the connector tube 23', its pressure drops gradually and the gas dissolved therein begins to bubble out.
Assume next that the reaction chamber 50' is operating under a reduced-pressure condition with P.sub.2 =0 kg/cm.sup.2 and P.sub.l =1 kg/cm.sup.2. In this situation, too, the pressure difference (.DELTA.P) is 1 kg cm.sup.2, but this causes not only the dissolved gas to bubble out but also the source liquid L' itself to vaporize. In other words, more bubbles are generated in this situation than when the reaction chamber 50' is operating under a normal-pressure condition. If the connector tube 23' is made longer, both the amount of the source liquid L' inside and the generation of bubbles increase accordingly. As the source liquid L' is supplied to the vaporization valve 12' and is vaporized therein, these bubbles burst and cause variations in the flow rate of the liquid L', as shown in FIG. 7.
FIG. 7 shows the time rate of change in the reaction pressure inside the reaction chamber 50'. As the vaporization valve 12' is opened, TEOS gas is carried by a carrier gas (H) and supplied into the reaction chamber 50', causing a sudden rise in the reaction pressure inside the chamber 50'. After the pressure reaches a certain level, it remains more or less at this level, rising again after a while and resulting in a sawtooth waveform. This sawtooth waveform is precisely the result of the variations in the pressure caused by the bursting of the bubbles mentioned above. If the vaporization valve 12' is closed about 4 minutes after it was opened, the reaction pressure inside the reaction chamber 50' decreases gradually first and then drops suddenly. Such variations in the reaction pressure inside the reaction chamber, caused by the bursting of the bubbles, have adverse effects on the semiconductor wafer processing.